Genes having small quantitative effects rather than major qualitative effects will be mapped and characterized. The hypothesis upon which this work is based suggests that "wild-type" genes are polymorphic and have varying but small effects on the phenotype of the individual carrying them. When they are mutant, these genes have major effects and show Mendelian inheritance. Experimental evidence supporting this idea is discussed. Maize is easily manipulated genetically and is a very polymorphic species and therefore lends itself to the characterization of quantitative trait loci. Two large recombinant inbred families have been used to construct a map that spans the entire genome. These families will be used first to map genetic factors affecting quantitative traits: recombination, disease resistance, plant characters, and response to the environment. Mapping should lead to the identification of genes which, when mutant, show Mendelian segregation. These genes will be molecularly cloned so that the correlation established by mapping can be proved by showing that the segregating "wild-type" alleles indeed have quantitative differences in their expression. While the characters studied here are of more interest to plant science and agronomy, the work has important implications for the study of quantitative factors of medical importance. Recombinant inbred lines have been established in mice, but maps associated with them are still incomplete. Mapping in human families likewise makes use of some of the same ideas that were established for recombinant inbred families. Maize is thus a model system where quantitative trait loci can more easily be mapped and characterized.